UNC-6/netrin and its receptor UNC-5 locally exclude presynaptic components from dendrites

UNC5C: Novel Gene Associated with Psychiatric Disorders Impacts Dysregulation of Axon Guidance Pathways

The UNC-5 family of netrin receptor genes, predominantly expressed in brain tissues, plays a pivotal role in various neuronal processes. Mutations in genes involved in axon development contribute to a wide spectrum of human diseases, including developmental, neuropsychiatric, and neurodegenerative disorders. The NTN1/DCC signaling pathway, interacting with UNC5C, plays a crucial role in central nervous system axon guidance and has been associated with psychiatric disorders during adolescence in humans. Whole-exome sequencing analysis unveiled two compound heterozygous causative mutations within the UNC5C gene in a patient diagnosed with psychiatric disorders. In silico analysis demonstrated that neither of the observed variants affected the allosteric linkage between UNC5C and NTN1. In fact, these mutations are located within crucial cytoplasmic domains, specifically ZU5 and the region required for the netrin-mediated axon repulsion of neuronal growth cones. These domains play a critical role in forming the supramodular protein structure and directly interact with microtubules, thereby ensuring the functionality of the axon repulsion process. We emphasize that these mutations disrupt the aforementioned processes, thereby associating the UNC5C gene with psychiatric disorders for the first time and expanding the number of genes related to psychiatric disorders. Further research is required to validate the correlation of the UNC5C gene with psychiatric disorders, but we suggest including it in the genetic analysis of patients with psychiatric disorders.

Cellular and Molecular Mechanisms Underlying Synaptic Subcellular Specificity

Chemical synapses are essential for neuronal information storage and relay. The synaptic signal received or sent from spatially distinct subcellular compartments often generates different outcomes due to the distance or physical property difference. Therefore, the final output of postsynaptic neurons is determined not only by the type and intensity of synaptic inputs but also by the synaptic subcellular location. How synaptic subcellular specificity is determined has long been the focus of study in the neurodevelopment field. Genetic studies from invertebrates such as Caenorhabditis elegans (C. elegans) have uncovered important molecular and cellular mechanisms required for subcellular specificity. Interestingly, similar molecular mechanisms were found in the mammalian cerebellum, hippocampus, and cerebral cortex. This review summarizes the comprehensive advances in the cellular and molecular mechanisms underlying synaptic subcellular specificity, focusing on studies from C. elegans and rodents.

Synaptogenesis: unmasking molecular mechanisms using Caenorhabditis elegans

The nematode Caenorhabditis elegans is a research model organism particularly suited to the mechanistic understanding of synapse genesis in the nervous system. Armed with powerful genetics, knowledge of complete connectomics, and modern genomics, studies using C. elegans have unveiled multiple key regulators in the formation of a functional synapse. Importantly, many signaling networks display remarkable conservation throughout animals, underscoring the contributions of C. elegans research to advance the understanding of our brain. In this chapter, we will review up-to-date information of the contribution of C. elegans to the understanding of chemical synapses, from structure to molecules and to synaptic remodeling.

Channel-independent function of UNC-9/Innexin in spatial arrangement of GABAergic synapses in C. elegans

Precise synaptic connection of neurons with their targets is essential for the proper functioning of the nervous system. A plethora of signaling pathways act in concert to mediate the precise spatial arrangement of synaptic connections. Here we show a novel role for a gap junction protein in controlling tiled synaptic arrangement in the GABAergic motor neurons in Caenorhabditis elegans, in which their axons and synapses overlap minimally with their neighboring neurons within the same class. We found that while EGL-20/Wnt controls axonal tiling, their presynaptic tiling is mediated by a gap junction protein UNC-9/Innexin, that is localized at the presynaptic tiling border between neighboring dorsal D-type GABAergic motor neurons. Strikingly, the gap junction channel activity of UNC-9 is dispensable for its function in controlling tiled presynaptic patterning. While gap junctions are crucial for the proper functioning of the nervous system as channels, our finding uncovered the novel channel-independent role of UNC-9 in synapse patterning.

The ANC-1 (Nesprin-1/2) organelle-anchoring protein functions through mitochondria to polarize axon growth in response to SLT-1

A family of giant KASH proteins, including C. elegans ANC-1 and mammalian Nesprin-1 and -2, are involved in organelle anchoring and are associated with multiple neurodevelopmental disorders including autism, bipolar disorder, and schizophrenia. However, little is known about how these proteins function in neurons. Moreover, the role of organelle anchoring in axon development is poorly understood. Here, we report that ANC-1 functions with the SLT-1 extracellular guidance cue to polarize ALM axon growth. This role for ANC-1 is specific to its longer ANC-1A and ANC-1C isoforms, suggesting that it is mechanistically distinct from previously described roles for ANC-1. We find that ANC-1 is required for the localization of a cluster of mitochondria to the base of the proximal axon. Furthermore, genetic and pharmacological studies indicate that ANC-1 functions with mitochondria to promote polarization of ALM axon growth. These observations reveal a mechanism whereby ANC-1 functions through mitochondria to polarize axon growth in response to SLT-1.

Neurexin and frizzled intercept axonal transport at microtubule minus ends to control synapse formation

Synapse formation is locally determined by transmembrane proteins, yet synaptic material is synthesized remotely and undergoes processive transport in axons. How local synaptogenic signals intercept synaptic cargo in transport to promote its delivery and synapse formation is unknown. We found that the control of synaptic cargo delivery at microtubule (MT) minus ends mediates pro- and anti-synaptogenic activities of presynaptic neurexin and frizzled in C. elegans and identified the atypical kinesin VAB-8/KIF26 as a key molecule in this process. VAB-8/KIF26 levels at synaptic MT minus ends are controlled by frizzled and neurexin; loss of VAB-8 mimics neurexin mutants or frizzled hyperactivation, and its overexpression can rescue synapse loss in these backgrounds. VAB-8/KIF26 is required for the synaptic localization of other minus-end proteins and promotes the pausing of retrograde transport to allow delivery to synapses. Consistently, reducing retrograde transport rescues synapse loss in vab-8 and neurexin mutants. These results uncover a mechanistic link between synaptogenic signaling and axonal transport.

The Immunoglobulin Superfamily Members syg-2 and syg-1 Regulate Neurite Development in C. elegans

Neurons form elaborate networks by guiding axons and dendrites to appropriate destinations. Neurites require information about the relative body axes during the initial projection from the cell body, and failure to receive or interpret those cues correctly can result in outgrowth errors. We identified a mutation in the Ig superfamily member syg-2 in a screen for animals with anterior/posterior (A/P) axon guidance defects. We found that syg-2 and its cognate Ig family member syg-1 appear to function in a linear genetic pathway to control the outgrowth of GABAergic axons. We determined that this pathway works in parallel to Wnt signaling. Specifically, mutations in syg-2 or syg-1 selectively affected the embryonically derived Dorsal D-type (DD) GABAergic neurons. We found no evidence that these mutations affected the Ventral D-type neurons (VD) that form later, during the first larval stage. In addition, mutations in syg-1 or syg-2 could result in the DD neurons forming multiple processes, becoming bipolar, rather than the expected pseudounipolar morphology. Given SYG-2′s essential function in synaptogenesis of the hermaphrodite-specific neurons (HSNs), we also examined DD neuron synapses in syg-2 mutants. We found syg-2 mutants had a decreased number of synapses formed, but synaptic morphology was largely normal. These results provide further evidence that the GABAergic motorneurons use multiple guidance pathways during development.

Molecular topography of an entire nervous system

We have produced gene expression profiles of all 302 neurons of the C. elegans nervous system that match the single-cell resolution of its anatomy and wiring diagram. Our results suggest that individual neuron classes can be solely identified by combinatorial expression of specific gene families. For example, each neuron class expresses distinct codes of ∼23 neuropeptide genes and ∼36 neuropeptide receptors, delineating a complex and expansive "wireless" signaling network. To demonstrate the utility of this comprehensive gene expression catalog, we used computational approaches to (1) identify cis-regulatory elements for neuron-specific gene expression and (2) reveal adhesion proteins with potential roles in process placement and synaptic specificity. Our expression data are available at https://cengen.org and can be interrogated at the web application CengenApp. We expect that this neuron-specific directory of gene expression will spur investigations of underlying mechanisms that define anatomy, connectivity, and function throughout the C. elegans nervous system.

Neurodevelopment: UNC-40/DCC and the Patterning of Neural Circuits

A new study in Caenorhabditis elegans suggests the ubiquitin-proteasome system promotes degradation of the netrin receptor UNC-40 in a particular neuron only in one sex, leading to sex-specific patterns of synaptic connections.

Establishment and Maintenance of Neural Circuit Architecture

The ability to sense the world, process information, and navigate the environment depends on the assembly and continuous function of neural circuits in the brain. Within the past two decades, new technologies have rapidly advanced our understanding of how neural circuits are wired during development and how they are stably maintained, often for years. Electron microscopy reconstructions of model organism connectomes have provided a map of the stereotyped (and variable) connections in the brain; advanced light microscopy techniques have enabled direct observation of the cellular dynamics that underlie circuit construction and maintenance; transcriptomic and proteomic surveys of both developing and mature neurons have provided insights into the molecular and genetic programs governing circuit establishment and maintenance; and advanced genetic techniques have allowed for high-throughput discovery of wiring regulators. These tools have empowered scientists to rapidly generate and test hypotheses about how circuits establish and maintain connectivity. Thus, the set of principles governing circuit formation and maintenance have been expanded. These principles are discussed in this review.

Temperature regulates synaptic subcellular specificity mediated by inhibitory glutamate signaling

Environmental factors such as temperature affect neuronal activity and development. However, it remains unknown whether and how they affect synaptic subcellular specificity. Here, using the nematode Caenorhabditis elegans AIY interneurons as a model, we found that high cultivation temperature robustly induces defects in synaptic subcellular specificity through glutamatergic neurotransmission. Furthermore, we determined that the functional glutamate is mainly released by the ASH sensory neurons and sensed by two conserved inhibitory glutamate-gated chloride channels GLC-3 and GLC-4 in AIY. Our work not only presents a novel neurotransmission-dependent mechanism underlying the synaptic subcellular specificity, but also provides a potential mechanistic insight into high-temperature-induced neurological defects.

The Netrin-1/DCC Guidance Cue Pathway as a Molecular Target in Depression: Translational Evidence

The Netrin-1/DCC guidance cue pathway plays a critical role in guiding growing axons toward the prefrontal cortex during adolescence and in the maturational organization and adult plasticity of prefrontal cortex connectivity. In this review, we put forward the idea that alterations in prefrontal cortex architecture and function, which are intrinsically linked to the development of major depressive disorder, originate in part from the dysregulation of the Netrin-1/DCC pathway by a mechanism that involves microRNA-218. We discuss evidence derived from mouse models of stress and from human postmortem brain and genome-wide association studies indicating an association between the Netrin-1/DCC pathway and major depressive disorder. We propose a potential role of circulating microRNA-218 as a biomarker of stress vulnerability and major depressive disorder.

Mitochondrial dynamics and bioenergetics regulated by netrin-1 in oligodendrocytes

Mitochondria are dynamic organelles that produce energy and molecular precursors that are essential for myelin synthesis. Unlike in neurons, mitochondria in oligodendrocytes increase intracellular movement in response to glutamatergic activation and are more susceptible to oxidative stress than in astrocytes or microglia. The signaling pathways that regulate these cell type-specific mitochondrial responses in oligodendrocytes are not understood. Here, we visualized mitochondria migrating through thin cytoplasmic channels crossing myelin basic protein-positive compacted membranes and localized within paranodal loop cytoplasm. We hypothesized that local extracellular enrichment of netrin-1 might regulate the recruitment and function of paranodal proteins and organelles, including mitochondria. We identified rapid recruitment of mitochondria and paranodal proteins, including neurofascin 155 (NF155) and the netrin receptor deleted in colorectal carcinoma (DCC), to sites of contact between oligodendrocytes and netrin-1-coated microbeads in vitro. We provide evidence that Src-family kinase activation and Rho-associated protein kinase (ROCK) inhibition downstream of netrin-1 induces mitochondrial elongation, hyperpolarization of the mitochondrial inner membrane, and increases glycolysis. Our findings identify a signaling mechanism in oligodendrocytes that is sufficient to locally recruit paranodal proteins and regulate the subcellular localization, morphology, and function of mitochondria.

Specificity of synapse formation in Aplysia: paracrine and autocrine signaling regulates bidirectional molecular interactions between sensory and non-target motor neurons

The formation of appropriate neural connections during development is critical for the proper wiring and functioning of the brain. Although considerable research suggests that the specificity of synapse formation is supported by complex intercellular signaling between potential presynaptic and postsynaptic partners, the extracellular factors and the intracellular signal transduction pathways engaged in this process remain largely unknown. Using the sensory-motor neural circuit that contributes to learning in defensive withdrawal reflexes in Aplysia californica, we investigated the molecular processes governing the interactions between sensory neurons and both target and non-target motor neurons during synapse formation in culture. We found that evolutionarily-conserved intercellular and intracellular signaling mechanisms critical for learning-related plasticity are also engaged during synaptogenesis in this in vitro model system. Our results reveal a surprising bidirectional regulation of molecular signaling between sensory neurons and non-target motor neurons. This regulation is mediated by signaling via both paracrine and autocrine diffusible factors that induce differential effects on transcription and on protein expression/activation in sensory neurons and in target and non-target motor neurons. Collectively, our data reveal novel molecular mechanisms that could underlie the repression of inappropriate synapse formation, and suggest mechanistic similarities between developmental and learning-related plasticity.

Molecular characterization of the netrin-1 UNC-5 receptor in Lucilia sericata larvae

Larval therapy with Lucilia sericata is a promising strategy in wound healing. Axon guidance molecules play vital roles during the development of the nervous system and also regulate the capacity of neuronal restoration in wound healing. Netrin-1, one of the proteins that larvae secrete, plays a useful role in cell migration and nerve tissue regeneration. The UNC-5 receptor combines with a netrin-1 signal and transmits the signal from one side of the membrane to the other side, initiating a change in cell activity. In the current study, we identified the full length of the UNC-5 receptor mRNA in L. sericata using different sets of primers, including exon junction and specific region primers. The coding sequence (CDS) of the UNC-5 receptor was sequenced and identified to include 633 base-pair nucleic acids, and BLAST analysis on its nucleotide sequence revealed 96% identity with the Lucilia cuprina netrin-1 UNC-5 receptor. The protein residue included 210 amino acids (aa) and coded for a protein with 24 kD weight. This gene lacked the signal peptide. Furthermore, the UPA domain is conserved in UNC-5. It lied at the interval of 26–131 aa. We identified the CDS of netrin-1UNC-5 receptor in L. sericata. It could be applied to research activities implementing a new essential component design in wound healing.

Intrinsic and extrinsic mechanisms of synapse formation and specificity in C. elegans

Precise neuronal wiring is critical for the function of the nervous system and is ultimately determined at the level of individual synapses. Neurons integrate various intrinsic and extrinsic cues to form synapses onto their correct targets in a stereotyped manner. In the past decades, the nervous system of nematode (Caenorhabditis elegans) has provided the genetic platform to reveal the genetic and molecular mechanisms of synapse formation and specificity. In this review, we will summarize the recent discoveries in synapse formation and specificity in C. elegans.

Regulation of Caenorhabditis elegans neuronal polarity by heterochronic genes

Many neurons display characteristic patterns of synaptic connections that are under genetic control. The Caenorhabditis elegans DA cholinergic motor neurons form synaptic connections only on their dorsal axons. We explored the genetic pathways that specify this polarity by screening for gene inactivations and mutations that disrupt this normal polarity of a DA motorneuron. A RAB-3::GFP fusion protein that is normally localized to presynaptic terminals along the dorsal axon of the DA9 motorneuron was used to screen for gene inactivations that disrupt the DA9 motorneuron polarity. This screen identified heterochronic genes as major regulators of DA neuron presynaptic polarity. In many heterochronic mutants, presynapses of this cholinergic motoneuron are mislocalized to the dendrite at the ventral side: inactivation of the blmp-1 transcription factor gene, the lin-29/Zn finger transcription factor, lin-28/RNA binding protein, and the let-7miRNA gene all disrupt the presynaptic polarity of this DA cholinergic neuron. We also show that the dre-1/F box heterochronic gene functions early in development to control maintenance of polarity at later stages, and that a mutation in the let-7 heterochronic miRNA gene causes dendritic misplacement of RAB-3 presynaptic markers that colocalize with muscle postsynaptic terminals ectopically. We propose that heterochronic genes are components in the UNC-6/Netrin pathway of synaptic polarity of these neurons. These findings highlight the role of heterochronic genes in postmitotic neuronal patterning events.

Drosophila Netrin-B controls mushroom body axon extension and regulates courtship-associated learning and memory of a Drosophila fragile X syndrome model

Mushroom body (MB) is a prominent structure essential for olfactory learning and memory in the Drosophila brain. The development of the MB involves the appropriate guidance of axon lobes and sister axon branches. Appropriate guidance that accurately shapes MB development requires the integration of various guidance cues provided by a series of cell types, which guide axons to reach their final positions within the MB neuropils. Netrins are axonal guidance molecules that are conserved regulators of embryonic nerve cord patterning. However, whether they contribute to MB morphogenesis has not yet been evaluated. Here, we find that Netrin-B (NetB) is highly expressed in the MB lobes, regulating lobe length through genetic interactions with the receptors Frazzled and Uncoordinated-5 from 24 h after pupal formation onwards. We observe that overexpression of NetB causes severe β lobe fusion in the MB, which is similar to the MB defects seen in the Drosophila model of fragile X syndrome (FXS). Our results further show that fragile-X mental retardation protein FMRP inhibits the translational activity of human ortholog Netrin-1 (NTN1). Knock-down of NetB significantly rescues the MB defects and ameliorates deficits in the learning and memory in FXS model Drosophila. These results indicate a critical role for NetB in MB lobe extension and identify NetB as a novel target of FMRP which contributes to learning and memory.

Branching mechanisms shaping dendrite architecture

A neuron's contribution to the information flow within a neural circuit is governed by the structure of its dendritic arbor. The geometry of the dendritic arbor directly determines synaptic density and the size of the receptive field, both of which influence the firing pattern of the neuron. Importantly, the position of individual dendritic branches determines the identity of the neuron's presynaptic partner and thus the nature of the incoming sensory information. To generate the unique stereotypic architecture of a given neuronal subtype, nascent branches must emerge from the dendritic shaft at preprogramed branch points. Subsequently, a complex array of extrinsic factors regulates the degree and orientation of branch expansion to ensure maximum coverage of the receptive field whilst constraining growth within predetermined territories. In this review we focus on studies that best illustrate how environmental cues such as the Wnts and Netrins and their receptors sculpt the dendritic arbor. We emphasize the pivotal role played by the actin cytoskeleton and its upstream regulators in branch initiation, outgrowth and navigation. Finally, we discuss how protocadherin and DSCAM contact-mediated repulsion prevents inappropriate synapse formation between sister dendrites or dendrites and the axon from the same neuron. Together these studies highlight the clever ways evolution has solved the problem of constructing complex branch geometries.

A perspective on SOAL, a stochastic model of neuronal outgrowth

A functional nervous system requires neuronal connections to be made in a highly detailed and stereotypic manner. During development, neurons extend processes that can branch, travel in different directions, and form elaborate patterns. These patterns are essential for forming proper connections. Patterns of outgrowth are produced by complex molecular events that cause a fluid membrane to move. The collective impact of dynamic fluctuating events at the microscale cause the patterns of outgrowth observed at the macroscale. Patterning is genetically controlled, but the effects genes have on membrane movement and patterning are not well understood. To better understand how genes control outgrowth patterns, I propose a statistically-oriented asymmetric localization (SOAL) model. This model is based on the theory that receptor-mediated outgrowth activity is stochastically oriented and when the system is at equilibrium there is an equal probability of outgrowth being oriented in any direction. This concept allows a statistical mechanics approach that can correlate the microscale events of outgrowth to the observed macroscale patterns. Proof-of-concept experiments suggest this approach can be used to study the effect genes have on outgrowth patterns. The SOAL model also provides a new theoretical framework for conceptualizing guidance. According to the model, outgrowth activity becomes asymmetrically localized to the neuron's surface in a statistically dependent manner. Extracellular cues regulate the probability of outgrowth along the surface and the orientation of outgrowth fluctuates across the surface over time. This creates a directional bias that allows the growth cone to navigate in reference to the composition of extracellular cues.

Aberrant information transfer interferes with functional axon regeneration

Functional axon regeneration requires regenerating neurons to restore appropriate synaptic connectivity and circuit function. To model this process, we developed an assay in Caenorhabditis elegans that links axon and synapse regeneration of a single neuron to recovery of behavior. After axon injury and regeneration of the DA9 neuron, synapses reform at their pre-injury location. However, these regenerated synapses often lack key molecular components. Further, synaptic vesicles accumulate in the dendrite in response to axon injury. Dendritic vesicle release results in information misrouting that suppresses behavioral recovery. Dendritic synapse formation depends on dynein and jnk-1. But even when information transfer is corrected, axonal synapses fail to adequately transmit information. Our study reveals unexpected plasticity during functional regeneration. Regeneration of the axon is not sufficient for the reformation of correct neuronal circuits after injury. Rather, synapse reformation and function are also key variables, and manipulation of circuit reformation improves behavioral recovery.

Rap2 and TNIK control Plexin-dependent tiled synaptic innervation in C. elegans

During development, neurons form synapses with their fate-determined targets. While we begin to elucidate the mechanisms by which extracellular ligand-receptor interactions enhance synapse specificity by inhibiting synaptogenesis, our knowledge about their intracellular mechanisms remains limited. Here we show that Rap2 GTPase (rap-2) and its effector, TNIK (mig-15), act genetically downstream of Plexin (plx-1) to restrict presynaptic assembly and to form tiled synaptic innervation in C. elegans. Both constitutively GTP- and GDP-forms of rap-2 mutants exhibit synaptic tiling defects as plx-1 mutants, suggesting that cycling of the RAP-2 nucleotide state is critical for synapse inhibition. Consistently, PLX-1 suppresses local RAP-2 activity. Excessive ectopic synapse formation in mig-15 mutants causes a severe synaptic tiling defect. Conversely, overexpression of mig-15 strongly inhibited synapse formation, suggesting that mig-15 is a negative regulator of synapse formation. These results reveal that subcellular regulation of small GTPase activity by Plexin shapes proper synapse patterning in vivo.

Genes do more than just direct the color of our hair or eyes. They produce proteins that are involved in almost every process in the body. In humans, the majority of active genes can be found in the brain, where they help it to develop and work properly – effectively controlling how we move and behave.

The brain’s functional units, the nerve cells or neurons, communicate with each other by releasing messenger molecules in the gap between them, the synapse. These molecules are then picked up from specific receptor proteins of the receiving neuron.

In the nervous system, neurons only form synapses with the cells they need to connect with, even though they are surrounded by many more cells. This implies that they use specific mechanisms to stop neurons from forming synapses with incorrect target cells. This is important, because if too many synapses were present or if synapses formed with incorrect target cells, it would compromise the information flow in the nervous system. This would ultimately lead to various neurological conditions, including Autism Spectrum Disorder.

In 2013, researchers found that in the roundworm Caenorhabditis elegans, a receptor protein called Plexin, is located at the surface of the neurons and can inhibit the formation of nearby synapses. Now, Chen et al. – including one author involved in the previous research – wanted to find out what genes Plexin manipulates when it stops synapses from growing. Knowing what each of those genes does can help us understand how neurons can inhibit synapses.

The results revealed that Plexin appears to regulate two genes, Rap2 and TNIK. Plexin reduced the activity of Rap2 in the neuron that released the messenger, which hindered the formation of synapses. The gene TNIK and its protein on the other hand, have the ability to modify other proteins and could so inhibit the growth of synapses. When TNIK was experimentally removed, the number of synapses increased, but when its activity was increased, the number of synapses was strongly reduced.

These findings could help scientists understand how mutations in Rap2 or TNIK can lead to various neurological conditions. A next step will be to test if these genes also affect the formation of synapses in other species such as mice, which have a more complex nervous system that is structurally and functionally more similar to that of humans.

Sexually Dimorphic unc-6/Netrin Expression Controls Sex-Specific Maintenance of Synaptic Connectivity

Nervous systems display intriguing patterns of sexual dimorphisms across the animal kingdom, but the mechanisms that generate such dimorphisms remain poorly characterized. In the nematode Caenorhabditis elegans, a number of neurons present in both sexes are synaptically connected to one another in a sexually dimorphic manner as a result of sex-specific synaptic pruning and maintenance [1-3]. We define here a mechanism for the male-specific maintenance of the synaptic connections of the phasmid sensory neuron PHB and its male-specific target, the sex-shared AVG interneuron. We show that the C. elegans Netrin ortholog UNC-6, signaling through its cognate receptor UNC-40/DCC and the CED-5/DOCK180 guanine nucleotide exchange factor, is both required and sufficient for male-specific synaptic maintenance. The dimorphism of unc-6 activity is brought about by sex-specific regulation of unc-6 transcription. Although unc-6 is transcribed in the AVG neuron of males and hermaphrodites during juvenile stages, unc-6 expression is downregulated in AVG in hermaphrodites during sexual maturation but is maintained during sexual maturation of males. unc-6 downregulation in hermaphrodites is conferred by the master regulator of hermaphrodite sexual identity, the Gli/CI homolog TRA-1, which antagonizes the non-sex-specific function of the LIM homeobox gene lin-11, a terminal selector and activator of unc-6 in AVG. Preventing the downregulation of unc-6 in AVG of hermaphrodites through ectopic expression of unc-6 in transgenic animals results in the maintenance of the PHB>AVG synapses in hermaphrodites. Taken together, intersectional transcriptional regulation of unc-6/Netrin is required and sufficient to cell autonomously pattern sexually dimorphic synapses.

Deep phenotyping unveils hidden traits and genetic relations in subtle mutants

Discovering mechanistic insights from phenotypic information is critical for the understanding of biological processes. For model organisms, unlike in cell culture, this is currently bottlenecked by the non-quantitative nature and perceptive biases of human observations, and the limited number of reporters that can be simultaneously incorporated in live animals. An additional challenge is that isogenic populations exhibit significant phenotypic heterogeneity. These difficulties limit genetic approaches to many biological questions. To overcome these bottlenecks, we developed tools to extract complex phenotypic traits from images of fluorescently labelled subcellular landmarks, using C. elegans synapses as a test case. By population-wide comparisons, we identified subtle but relevant differences inaccessible to subjective conceptualization. Furthermore, the models generated testable hypotheses of how individual alleles relate to known mechanisms or belong to new pathways. We show that our model not only recapitulates current knowledge in synaptic patterning but also identifies novel alleles overlooked by traditional methods.

Experimenter scoring of cellular imaging data can be biased. This study describes an automated and unbiased multidimensional phenotyping method that relies on machine learning and complex feature computation of imaging data, and identifies weak alleles affecting synapse morphology in live C. elegans.

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

The correct wiring of neuronal circuits depends on outgrowth and guidance of neuronal processes during development. In the past two decades, great progress has been made in understanding the molecular basis of axon outgrowth and guidance. Genetic analysis in Caenorhabditis elegans has played a key role in elucidating conserved pathways regulating axon guidance, including Netrin signaling, the slit Slit/Robo pathway, Wnt signaling, and others. Axon guidance factors were first identified by screens for mutations affecting animal behavior, and by direct visual screens for axon guidance defects. Genetic analysis of these pathways has revealed the complex and combinatorial nature of guidance cues, and has delineated how cues guide growth cones via receptor activity and cytoskeletal rearrangement. Several axon guidance pathways also affect directed migrations of non-neuronal cells in C. elegans, with implications for normal and pathological cell migrations in situations such as tumor metastasis. The small number of neurons and highly stereotyped axonal architecture of the C. elegans nervous system allow analysis of axon guidance at the level of single identified axons, and permit in vivo tests of prevailing models of axon guidance. C. elegans axons also have a robust capacity to undergo regenerative regrowth after precise laser injury (axotomy). Although such axon regrowth shares some similarities with developmental axon outgrowth, screens for regrowth mutants have revealed regeneration-specific pathways and factors that were not identified in developmental screens. Several areas remain poorly understood, including how major axon tracts are formed in the embryo, and the function of axon regeneration in the natural environment.

Regulation of neuronal axon specification by glia-neuron gap junctions in C. elegans

Axon specification is a critical step in neuronal development, and the function of glial cells in this process is not fully understood. Here, we show that C. elegans GLR glial cells regulate axon specification of their nearby GABAergic RME neurons through GLR-RME gap junctions. Disruption of GLR-RME gap junctions causes misaccumulation of axonal markers in non-axonal neurites of RME neurons and converts microtubules in those neurites to form an axon-like assembly. We further uncover that GLR-RME gap junctions regulate RME axon specification through activation of the CDK-5 pathway in a calcium-dependent manner, involving a calpain clp-4. Therefore, our study reveals the function of glia-neuron gap junctions in neuronal axon specification and shows that calcium originated from glial cells can regulate neuronal intracellular pathways through gap junctions.

Dependence receptor involvement in subtilisin-induced long-term depression and in long-term potentiation

The serine protease subtilisin induces a form of long-term depression (LTD) which is accompanied by a reduced expression of the axo-dendritic guidance molecule Unco-ordinated-5C (Unc-5C). One objective of the present work was to determine whether a loss of Unc-5C function contributed to subtilisin-induced LTD by using Unc-5C antibodies in combination with the pore-forming agents Triton X-100 (0.005%) or streptolysin O in rat hippocampal slices. In addition we have assessed the effect of subtilisin on the related dependence receptor Deleted in Colorectal Cancer (DCC) and used antibodies to this protein for functional studies. Field excitatory postsynaptic potentials (fEPSPs) were analyzed in rat hippocampal slices and protein extracts were used for Western blotting. Subtilisin produced a greater loss of DCC than of Unc-5C, but the antibodies had no effect on resting excitability or fEPSPs and did not modify subtilisin-induced LTD. However, antibodies to DCC but not Unc-5C did reduce the amplitude of theta-burst long-term potentiation (LTP). In addition, two inhibitors of endocytosis - dynasore and tat-gluR2(3Y) - were tested and, although the former compound had no effect on neurophysiological responses, tat-gluR2(3Y) did reduce the amplitude of subtilisin-induced LTD without affecting the expression of DCC or Unc-5C but with some loss of PostSynaptic Density Protein-95. The results support the view that the dependence receptor DCC may be involved in LTP and suggest that the endocytotic removal of a membrane protein or proteins may contribute to subtilisin-induced LTD, although it appears that neither Unc-5C nor DCC are involved in this process.

Pioneer Axon Navigation Is Controlled by AEX-3, a Guanine Nucleotide Exchange Factor for RAB-3 in Caenorhabditis elegans

Precise and accurate axon tract formation is an essential aspect of brain development. This is achieved by the migration of early outgrowing axons (pioneers) allowing later outgrowing axons (followers) to extend toward their targets in the embryo. In Caenorhabditis elegans the AVG neuron pioneers the right axon tract of the ventral nerve cord, the major longitudinal axon tract. AVG is essential for the guidance of follower axons and hence organization of the ventral nerve cord. In an enhancer screen for AVG axon guidance defects in a nid-1/Nidogen mutant background, we isolated an allele of aex-3 aex-3 mutant animals show highly penetrant AVG axon navigation defects. These defects are dependent on a mutation in nid-1/Nidogen, a basement membrane component. Our data suggest that AEX-3 activates RAB-3 in the context of AVG axon navigation. aex-3 genetically acts together with known players of vesicular exocytosis: unc-64/Syntaxin, unc-31/CAPS, and ida-1/IA-2. Furthermore our genetic interaction data suggest that AEX-3 and the UNC-6/Netrin receptor UNC-5 act in the same pathway, suggesting AEX-3 might regulate the trafficking and/or insertion of UNC-5 at the growth cone to mediate the proper guidance of the AVG axon.

The Caenorhabditis elegans Q neuroblasts: A powerful system to study cell migration at single-cell resolution in vivo

During development, cell migration plays a central role in the formation of tissues and organs. Understanding the molecular mechanisms that drive and control these migrations is a key challenge in developmental biology that will provide important insights into disease processes, including cancer cell metastasis. In this article, we discuss the Caenorhabditis elegans Q neuroblasts and their descendants as a tool to study cell migration at single-cell resolution in vivo. The highly stereotypical migration of these cells provides a powerful system to study the dynamic cytoskeletal processes that drive migration as well as the evolutionarily conserved signaling pathways (including different Wnt signaling cascades) that guide the cells along their specific trajectories. Here, we provide an overview of what is currently known about Q neuroblast migration and highlight the live-cell imaging, genome editing, and quantitative gene expression techniques that have been developed to study this process.

The PHR proteins: intracellular signaling hubs in neuronal development and axon degeneration

During development, a coordinated and integrated series of events must be accomplished in order to generate functional neural circuits. Axons must navigate toward target cells, build synaptic connections, and terminate outgrowth. The PHR proteins (consisting of mammalian Phr1/MYCBP2, Drosophila Highwire and C. elegans RPM-1) function in each of these events in development. Here, we review PHR function across species, as well as the myriad of signaling pathways PHR proteins regulate. These findings collectively suggest that the PHR proteins are intracellular signaling hubs, a concept we explore in depth. Consistent with prominent developmental functions, genetic links have begun to emerge between PHR signaling networks and neurodevelopmental disorders, such as autism, schizophrenia and intellectual disability. Finally, we discuss the recent and important finding that PHR proteins regulate axon degeneration, which has further heightened interest in this fascinating group of molecules.

Connectomics, the Final Frontier

How the synaptic connections in the nervous system are genetically encoded and formed during development remains an unsolved problem. The known connectivity of the nervous system of the nematode C. elegans provides an opportunity to search for the genes involved. The circuits for male mating behavior form a complex neural network that would seem to require a large family of molecular cell labels for pre- and postsynaptic cell recognition. It is suggested that a combinatorial code of neural cell adhesion proteins specifying the network of connections may be discovered by comparing the expression patterns of candidate genes with the pattern of connections.

Orchestrating A/P and D/V guidance - A Wnt/Netrin tale

While ample information was gathered in identifying guidance cues and their downstream mediators, very little is known about how the information from multiple extracellular cues is intracellularly to generate normal patterning. Netrin and Wnt signaling pathways play key roles in normal development as well as in malignancies. In C. elegans, as in vertebrates, dorso-ventral (D/V) graded distributions of UNC-6/Netrin and antero-posterior (A/P) graded distributions of Wnts provide instructive polarity information to guide cells and axons along their respective gradients. In this commentary, I will discuss recent findings demonstrating that these 2 signaling pathways also function redundantly to regulate polarity orthogonal to the axis of their gradation. Thus, Wnt signaling components contribute to D/V polarity, while Netrin signaling components contribute to A/P polarity and their joint action collaboratively governs migratory transitions from one axis to the other. These findings pave the way to unraveling broader roles of Wnt and Netrin signaling pathways, roles that are masked due to their redundant nature, and provide a conceptually novel view of how antero-posterior and dorso-ventral guidance mechanisms are orchestrated to establish polarity in multiple biological processes.

Netrin-1 directs dendritic growth and connectivity of vertebrate central neurons in vivo

BACKGROUND

Netrins are a family of extracellular proteins that function as chemotropic guidance cues for migrating cells and axons during neural development. In the visual system, netrin-1 has been shown to play a key role in retinal ganglion cell (RGC) axon growth and branching at the target, where presynaptic RGC axons form partnerships with the dendrites of tectal neurons. However, the signals that guide the connections between RGC axons and their postsynaptic partners are yet unknown. Here, we explored dynamic cellular mechanisms by which netrin-1 influences visual circuit formation, particularly those that impact postsynaptic neuronal morphology and connectivity during retinotectal wiring.

RESULTS

Time-lapse in vivo imaging of individual Xenopus laevis optic tectal neurons co-expressing tdTomato and PSD95-GFP revealed rapid remodeling and reorganization of dendritic arbors following acute manipulations in netrin-1 levels. Effects of altered netrin signaling on developing dendritic arbors of tectal neurons were distinct from its effects on presynaptic RGC axons. Within 4 h of treatment, tectal injection of recombinant netrin-1 or sequestration of endogenous netrin with an UNC-5 receptor ectodomain induced significant changes in the directionality and orientation of dendrite growth and in the maintenance of already established dendrites, demonstrating that relative levels of netrin are important for these functions. In contrast, altering DCC-mediated netrin signaling with function-blocking antibodies induced postsynaptic specialization remodeling and changed growth directionality of already established dendrites. Reducing netrin signaling also decreased avoidance behavior in a visually guided task, suggesting that netrin is essential for emergent visual system function.

CONCLUSIONS

These in vivo findings together with the patterns of expression of netrin and its receptors reveal an important role for netrin in the early growth and guidance of vertebrate central neuron dendritic arbors. Collectively, our studies indicate that netrin shapes both pre- and postsynaptic arbor morphology directly and in multiple ways at stages critical for functional visual system development.

Advances in synapse formation: forging connections in the worm

Synapse formation is the quintessential process by which neurons form specific connections with their targets to enable the development of functional circuits. Over the past few decades, intense research efforts have identified thousands of proteins that localize to the pre- and postsynaptic compartments. Genetic dissection has provided important insights into the nexus of the molecular and cellular network, and has greatly advanced our knowledge about how synapses form and function physiologically. Moreover, recent studies have highlighted the complex regulation of synapse formation with the identification of novel mechanisms involving cell interactions from non-neuronal sources. In this review, we cover the conserved pathways required for synaptogenesis and place specific focus on new themes of synapse modulation arising from studies in Caenorhabditis elegans. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

Cellular and molecular mechanisms of synaptic specificity

Precise connectivity in neuronal circuits is a prerequisite for proper brain function. The dauntingly complex environment encountered by axons and dendrites, even after navigation to their target area, prompts the question of how specificity of synaptic connections arises during development. We review developmental strategies and molecular mechanisms that are used by neurons to ensure their precise matching of pre- and postsynaptic elements. The emerging theme is that each circuit uses a combination of simple mechanisms to achieve its refined, often complex connectivity pattern. At increasing levels of resolution, from lamina choice to subcellular targeting, similar signaling concepts are reemployed to narrow the choice of potential matches. Temporal control over synapse development and synapse elimination further ensures the specificity of connections in the nervous system.

Subdomain-mediated axon-axon signaling and chemoattraction cooperate to regulate afferent innervation of the lateral habenula

A dominant feature of neural circuitry is the organization of neuronal projections and synapses into specific brain nuclei or laminae. Lamina-specific connectivity is controlled by the selective expression of extracellular guidance and adhesion molecules in the target field. However, how (sub)nucleus-specific connections are established and whether axon-derived cues contribute to subdomain targeting are largely unknown. Here, we demonstrate that the lateral subnucleus of the habenula (lHb) determines its own afferent innervation by sending out efferent projections that express the cell adhesion molecule LAMP to reciprocally collect and guide dopaminergic afferents to the lHb-a phenomenon we term subdomain-mediated axon-axon signaling. This process of reciprocal axon-axon interactions cooperates with lHb-specific chemoattraction mediated by Netrin-1, which controls axon target entry, to ensure specific innervation of the lHb. We propose that cooperation between pretarget reciprocal axon-axon signaling and subdomain-restricted instructive cues provides a highly precise and general mechanism to establish subdomain-specific neural circuitry.

Netrins and Wnts function redundantly to regulate antero-posterior and dorso-ventral guidance in C. elegans

Guided migrations of cells and developing axons along the dorso-ventral (D/V) and antero-posterior (A/P) body axes govern tissue patterning and neuronal connections. In C. elegans, as in vertebrates, D/V and A/P graded distributions of UNC-6/Netrin and Wnts, respectively, provide instructive polarity information to guide cells and axons migrating along these axes. By means of a comprehensive genetic analysis, we found that simultaneous loss of Wnt and Netrin signaling components reveals previously unknown and unexpected redundant roles for Wnt and Netrin signaling pathways in both D/V and A/P guidance of migrating cells and axons in C. elegans, as well as in processes essential for organ function and viability. Thus, in addition to providing polarity information for migration along the axis of their gradation, Wnts and Netrin are each able to guide migrations orthogonal to the axis of their gradation. Netrin signaling not only functions redundantly with some Wnts, but also counterbalances the effects of others to guide A/P migrations, while the involvement of Wnt signaling in D/V guidance identifies Wnt signaling as one of the long sought mechanisms that functions in parallel to Netrin signaling to promote D/V guidance of cells and axons. These findings provide new avenues for deciphering how A/P and D/V guidance signals are integrated within the cell to establish polarity in multiple biological processes, and implicate broader roles for Netrin and Wnt signaling - roles that are currently masked due to prevalent redundancy.

While ample information was gathered in past decades on identifying guidance cues and their downstream mediators, very little is known about how the information from multiple extracellular cues is integrated within the cell to generate normal patterning. Netrin and Wnt signaling pathways are both critical to multiple developmental processes and play key roles in normal development as well as in malignancies. The UNC-6/Netrin guidance cue has a conserved role in guiding cell and growth cone migrations along the dorso-ventral axis, whereas Wnts are critical for determining polarity and guidance along the antero-posterior axis. In this study we show that these two signaling pathways function redundantly in both antero-posterior and dorso-ventral guidance as well as in processes essential for viability. Furthermore, we demonstrate that a fine balance between Wnt and Netrin signaling pathways is critical for proper polarity establishment and identify Wnt signaling as one of the long sought mechanisms that signal in parallel to Netrin to promote dorso-ventral guidance of cells and axons in Caenorhabditis elegans. These findings pave the way to unraveling the broader roles of Wnt and Netrin signaling pathways and provide a conceptually novel view of how antero-posterior and dorso-ventral guidance mechanisms are orchestrated.

Extracellular matrix regulates UNC-6 (netrin) axon guidance by controlling the direction of intracellular UNC-40 (DCC) outgrowth activity

How extracellular molecules influence the direction of axon guidance is poorly understood. The HSN axon of Caenorhabditis elegans is guided towards a ventral source of secreted UNC-6 (netrin). The axon's outgrowth response to UNC-6 is mediated by the UNC-40 (DCC) receptor. We have proposed that in response to the UNC-6 molecule the direction of UNC-40-mediated axon outgrowth is stochastically determined. The direction of guidance is controlled by asymmetric cues, including the gradient of UNC-6, that regulate the probability that UNC-40-mediated axon outgrowth is directed on average, over time, in a specific direction. Here we provide genetic evidence that a specialized extracellular matrix, which lies ventral to the HSN cell body, regulates the probability that UNC-40-mediated axon outgrowth will be directed ventrally towards the matrix. We show that mutations that disrupt the function of proteins associated with this matrix, UNC-52 (perlecan), UNC-112 (kindlin), VAB-19 (Kank), and UNC-97 (PINCH), decrease the probability of UNC-40-mediated axon outgrowth in the ventral direction, while increasing the probability of outgrowth in the anterior and posterior directions. Other results suggest that INA-1 (α integrin) and MIG-15 (NIK kinase) signaling mediate the response in HSN. Although the AVM axon also migrates through this matrix, the mutations have little effect on the direction of AVM axon outgrowth, indicating that responses to the matrix are cell-specific. Together, these results suggest that an extracellular matrix can regulate the direction of UNC-6 guidance by increasing the probability that UNC-40-mediated axon outgrowth activity will be oriented in a specific direction.

RPM-1 uses both ubiquitin ligase and phosphatase-based mechanisms to regulate DLK-1 during neuronal development

The Pam/Highwire/RPM-1 (PHR) proteins are key regulators of neuronal development that function in axon extension and guidance, termination of axon outgrowth, and synapse formation. Outside of development, the PHR proteins also regulate axon regeneration and Wallerian degeneration. The PHR proteins function in part by acting as ubiquitin ligases that degrade the Dual Leucine zipper-bearing Kinase (DLK). Here, we show that the Caenorhabditis elegans PHR protein, Regulator of Presynaptic Morphology 1 (RPM-1), also utilizes a phosphatase-based mechanism to regulate DLK-1. Using mass spectrometry, we identified Protein Phosphatase Magnesium/Manganese dependent 2 (PPM-2) as a novel RPM-1 binding protein. Genetic, transgenic, and biochemical studies indicated that PPM-2 functions coordinately with the ubiquitin ligase activity of RPM-1 and the F-box protein FSN-1 to negatively regulate DLK-1. PPM-2 acts on S874 of DLK-1, a residue implicated in regulation of DLK-1 binding to a short, inhibitory isoform of DLK-1 (DLK-1S). Our study demonstrates that PHR proteins function through both phosphatase and ubiquitin ligase mechanisms to inhibit DLK. Thus, PHR proteins are potentially more accurate and sensitive regulators of DLK than originally thought. Our results also highlight an important and expanding role for the PP2C phosphatase family in neuronal development.

The molecular mechanisms that govern formation of functional synaptic connections are central to brain development and function. We have used the nematode C. elegans to explore the mechanism of how the intracellular signaling protein RPM-1 regulates neuronal development. Using a combination of proteomic, genetic, transgenic, and biochemical approaches we have shown that RPM-1 functions through a PP2C phosphatase, PPM-2, to regulate the activity of a MAP3 kinase, DLK-1. Our results indicate that a combination of PPM-2 phosphatase activity and RPM-1 ubiquitin ligase activity inhibit DLK-1.

The BAI subfamily of adhesion GPCRs: synaptic regulation and beyond

The brain-specific angiogenesis inhibitors 1-3 (BAI1-3) comprise a subfamily of adhesion G-protein-coupled receptors (GPCRs). These receptors are highly expressed in the brain and were first studied for their ability to inhibit angiogenesis and tumor formation. Subsequently, BAI1 was found to play roles in apoptotic cell phagocytosis and myoblast fusion. Until recently, however, little was known about the physiological importance of the BAI subfamily in the context of normal brain function. Recent work has provided evidence for key roles of BAI1-3 in the regulation of synaptogenesis and dendritic spine formation. In this review, we summarize the current understanding of the BAI subfamily with regard to downstream signaling pathways, physiological actions, and potential importance as novel drug targets in the treatment of psychiatric and neurological diseases.

Netrin-1 promotes excitatory synaptogenesis between cortical neurons by initiating synapse assembly

Netrin-1 is a secreted protein that directs long-range axon guidance during early stages of neural circuit formation and continues to be expressed in the mammalian forebrain during the postnatal period of peak synapse formation. Here we demonstrate a synaptogenic function of netrin-1 in rat and mouse cortical neurons and investigate the underlying mechanism. We report that netrin-1 and its receptor DCC are widely expressed by neurons in the developing mammalian cortex during synapse formation and are enriched at synapses in vivo. We detect DCC protein distributed along the axons and dendrites of cultured cortical neurons and provide evidence that newly translated netrin-1 is selectively transported to dendrites. Using gain and loss of function manipulations, we demonstrate that netrin-1 increases the number and strength of excitatory synapses made between developing cortical neurons. We show that netrin-1 increases the complexity of axon and dendrite arbors, thereby increasing the probability of contact. At sites of contact, netrin-1 promotes adhesion, while locally enriching and reorganizing the underlying actin cytoskeleton through Src family kinase signaling and m-Tor-dependent protein translation to locally cluster presynaptic and postsynaptic proteins. Finally, we demonstrate using whole-cell patch-clamp electrophysiology that netrin-1 increases the frequency and amplitude of mEPSCs recorded from cortical pyramidal neurons. These findings identify netrin-1 as a synapse-enriched protein that promotes synaptogenesis between mammalian cortical neurons.

Cell biology in neuroscience: cellular and molecular mechanisms underlying presynapse formation

Synapse formation is a highly regulated process that requires the coordination of many cell biological events. Decades of research have identified a long list of molecular components involved in assembling a functioning synapse. Yet how the various steps, from transporting synaptic components to adhering synaptic partners and assembling the synaptic structure, are regulated and precisely executed during development and maintenance is still unclear. With the improvement of imaging and molecular tools, recent work in vertebrate and invertebrate systems has provided important insight into various aspects of presynaptic development, maintenance, and trans-synaptic signals, thereby increasing our understanding of how extrinsic organizers and intracellular mechanisms contribute to presynapse formation.

Two Wnts instruct topographic synaptic innervation in C. elegans

Gradients of topographic cues play essential roles in the organization of sensory systems by guiding axonal growth cones. Little is known about whether there are additional mechanisms for precise topographic mapping of synaptic connections. Whereas the C. elegans DA8 and DA9 neurons have similar axonal trajectories, their synapses are positioned in distinct but adjacent domains in the anterior-posterior axis. We found that two Wnts, LIN-44 and EGL-20, are responsible for this spatial organization of synapses. Both Wnts form putative posterior-high, anterior-low gradients. The posteriorly expressed LIN-44 inhibits synapse formation in both DA9 and DA8, and creates a synapse-free domain on both axons via LIN-17 /Frizzled. EGL-20, a more anteriorly expressed Wnt, inhibits synapse formation through MIG-1/Frizzled, which is expressed in DA8 but not in DA9. The Wnt-Frizzled specificity and selective Frizzled expression dictate the stereotyped, topographic positioning of synapses between these two neurons.

Growth factors in synaptic function

Synapses are increasingly recognized as key structures that malfunction in disorders like schizophrenia, mental retardation, and neurodegenerative diseases. The importance and complexity of the synapse has fuelled research into the molecular mechanisms underlying synaptogenesis, synaptic transmission, and plasticity. In this regard, neurotrophic factors such as netrin, Wnt, transforming growth factor-β (TGF-β), tumor necrosis factor-α (TNF-α), and others have gained prominence for their ability to regulate synaptic function. Several of these factors were first implicated in neuroprotection, neuronal growth, and axon guidance. However, their roles in synaptic development and function have become increasingly clear, and the downstream signaling pathways employed by these factors have begun to be elucidated. In this review, we will address the role of these factors and their downstream effectors in synaptic function in vivo and in cultured neurons.

The cell biology of synaptic specificity during development

Proper circuit connectivity is critical for nervous system function. Connectivity derives from the interaction of two interdependent modules: synaptic specificity and synaptic assembly. Specificity involves both targeting of neurons to specific laminar regions and the formation of synapses onto defined subcellular areas. In this review, we focus discussion on recently elucidated molecular mechanisms that control synaptic specificity and link them to synapse assembly. We use these molecular pathways to underscore fundamental cell biological concepts that underpin, and help explain, the rules governing synaptic specificity.

unc5c haploinsufficient phenotype: striking similarities with the dcc haploinsufficiency model

DCC and UNC5 homologs (UNC5H) are guidance cue receptors highly expressed by mesocorticolimbic dopamine neurons. We have shown that dcc heterozygous mice exhibit increased dopamine, but not norepinephrine, innervation and function in medial prefrontal cortex. Concomitantly, dcc heterozygotes show blunted mesolimbic dopamine release and behavioral responses to stimulant drugs. These changes appear only in adulthood. Recently, we found an adolescent emergence of UNC5H expression by dopamine neurons and co-expression of DCC and UNC5H by single dopamine cells. Here, we demonstrate selective expression of unc5 homolog c mRNA by dopamine neurons in adulthood. We show that unc5c haploinsufficiency results in diminished amphetamine-induced locomotion in male and female mice. This phenotype is identical to that produced by dcc haploinsufficiency and is observed after adolescence. Notably, and similar to dcc haploinsufficiency, unc5c haploinsufficiency leads to dramatic increases in tyrosine hydroxylase expression in the medial prefrontal cortex, but not in the nucleus accumbens. In contrast, medial prefrontal cortex dopamine-β-hydroxylase expression is not altered. We confirmed that UNC5C protein is reduced in the ventral tegmental area of unc5c heterozygous mice, but that DCC expression in this region remains unchanged. UNC5C receptors may also play a role in dopamine function and influence sensitivity to behavioral effects of stimulant drugs of abuse, at least upon first exposure. The striking similarities between the dcc and the unc5c haploinsufficient phenotypes raise the possibility that functions mediated by DCC/UNC5C complexes may be at play.

Kinesin-1 acts with netrin and DCC to maintain sensory neuron position in Caenorhabditis elegans

The organization of neurons and the maintenance of that arrangement are critical to brain function. Failure of these processes in humans can lead to severe birth defects, mental retardation, and epilepsy. Several kinesins have been shown to play important roles in cell migration in vertebrate systems, but few upstream and downstream pathway members have been identified. Here, we utilize the genetic model organism Caenorhabditis elegans to elucidate the pathway by which the C. elegans Kinesin-1 Heavy Chain (KHC)/KIF5 ortholog UNC-116 functions to maintain neuronal cell body position in the PHB sensory neurons. We find that UNC-116/KHC acts in part with the cell and axon migration molecules UNC-6/Netrin and UNC-40/DCC in this process, but in parallel to SAX-3/Robo. We have also identified several potential adaptor, cargo, and regulatory proteins that may provide insight into the mechanism of UNC-116/KHC's function in this process. These include the cargo receptor UNC-33/CRMP2, the cargo adaptor protein UNC-76/FEZ and its regulator UNC-51/ULK, the cargo molecule UNC-69/SCOCO, and the actin regulators UNC-44/Ankyrin and UNC-34/Enabled. These genes also act in cell migration and axon outgrowth; however, many proteins that function in these processes do not affect PHB position. Our findings suggest an active posterior cell migration mediated by UNC-116/KHC occurs throughout development to maintain proper PHB cell body position and define a new pathway that mediates maintenance of neuronal cell body position.